This invention relates to a changeover valve adapted for use in a hydraulic system, such as an automotive brake system, which is required to delicately control the flow of high-pressure, low-viscosity hydraulic fluid within a wide range of flow rates, and a flow control valve assembly including the changeover valve.
FIG. 4 shows a conventional flow control valve assembly having a housing 1 formed with an inlet port 2 and an outlet port 3, and a spool 4 substantially liquid-tightly and slidably mounted in the housing 1. A first fluid chamber 5 and a second fluid chamber 6, which exert fluid pressure on both ends of the spool 4, communicate with each other through a fluid passage 8 having an orifice 7. A valve portion 9 is formed by a land portion formed on the inner surface of the housing 1 and a land portion formed on the outer surface of the spool which are brought into opposed position relative to each other. The fluid passage in the valve is opened and closed by the valve portion 9. The spool is biased by a biasing means 10. Thus, the controlled flow rate in the valve depends upon the biasing force of the biasing means 10.
In this control valve assembly, the spool 4 is biased by the biasing means 10 in a direction opposite to the direction of force applied to the spool by the pressure difference between the first and second fluid chambers 5 and 6. The spool 4 tends to move toward a point at which the pressures on both ends thereof balance with each other. Thus, the degree of opening of the valve portion 9 is automatically adjusted so that the above-mentioned pressure difference balances with the force of the biasing means 10. As a result, hydraulic fluid flows through the second fluid chamber 6 to the outlet port 3 at a rate determined by the pressure difference and the sectional area of the orifice 7.
If more accurate fluid flow control is desired, as shown in FIG. 5, the valve portion 9 may be formed by an elastic annular seal 11 provided around the outer periphery of the spool and a shoulder formed on the outer periphery of the spool and adapted to be brought into contact with the seal 11. This arrangement is disclosed in Unexamined Japanese Patent Publication 3-223578 filed by the applicant of the present invention.
One problem with the flow control valve assembly of FIG. 4 is that a certain degree of fluid leakage is inevitable, because some degree of gap is inevitably, formed between the land portion on the housing inner surface and the land portion on the spool outer surface. Such leakage results in errors in flow rate. Thus, this type of valve is not suitable for use in a hydraulic system used to control the flow rate of a low-viscosity, high-pressure hydraulic fluid with high accuracy.
The flow control valve assembly shown in FIG. 5 is free of fluid leakage because the spool shoulder is brought into tight contact with the elastic annular seal 11. But if a clearance 12 between the outer periphery of the spool 4 and the inner periphery of the housing 1, which is a part of the hydraulic fluid passage, is too wide, the seal 11 may get caught in the clearance and damaged. Thus, the clearance 12 has to be narrow. But, with a narrow clearance 12, the valve cannot pass a large flow rate. Such a control valve assembly has limited use.